In the US large Turbojet aircraft are not allowed to operate below the Class B floor (or outside its lateral extent) when operating to the airport for which the Class B was established - so they must contain all of the missed approach tracks, SIDs and STARs (which on a casual glance appear to be equally complex to those of LHR). Conversely, LHR would seem to need more holding capacity. I suspect this is primarily due to exceptionally high runway utilisation, aggravated by a European ATC infrastructure that is not particularly joined up, and, I suspect, an inability to coordinate speeds of inbound long haul aircraft (the later two making flow control less effective than it seems to be in the US).
Most US Class Bs would have a 1500 ft shelf allowing the BUR-ASCOT transition without bothering LHR-SVFR (but of course with mandatory Mode-C). I have been less successful on this particular transition, probably only getting it 50% of the time. I am told this is just luck of the draw in that it can only be used one direction at a time. In addition, many of the places like WW, Denham, Fairoaks would, in the US, have some breathing space around them due to the smaller footprint of the inner ring (that goes down to SFC) of the Class B.